Production of x-ray intensifying screens

ABSTRACT

A base is coated with a dispersion of a fluorescent salt in a solution of a polymeric binder in a low-boiling point and a highboiling point organic solvent; while this layer is still moist, a transparent film-forming polymeric material dissolvent in a solvent is coated onto it and then the screen is dried.

ite tates atent Linturn Oct. 1, 1974 [541 PRODUCTION OF X-RAY INTENSIFYING 2,907,882 10 1959 Patten 117 335 R SCREENS 3,043,710 7/1962 Patten ct a1. 117/335 R [75] Inventor: Patrick John Linturn, llford,

England Primary ExaminerWilliam D. Martin Assistant ExaminerWilliam R. Trenor [73] Amgnee' ford Limited Essex Attorney, Agent, or Firm--Wenderoth, Lind & Ponack England [22] Filed: June 2, 1972 21 Appl. No.: 259,302 [57] A A T A base is coated with a dispersion of a fluorescent salt [52] US. Cl. 117/335 R, 117/76 R, 250/80, in a Solution of a polymeric binder in a lowboiling 250/473, 250/483, 250/487, 250/488 point and a high-boiling point organic solvent; while [51] Int. Cl. ..C09k 1/00, H01] 31/20 this layer is Still moist, a transparent film forming [58] Field of Search ..1l7/33.5 R, polymeric material dissolvent in a solvent is coated 117/76 R; 250/487, 483 onto it and then the screen is dried.

[56] References Cited 6 Claims, N0 Drawings UNITED STATES PATENTS 8/1953 Smith 117/335 R PRODUCTION OF X-RAY INTENSIFYING SCREENS X-ray intensifying screens usually comprise a dimensionally stable support base there being coated thereon, in order, a light reflecting layer, a fluorescent layer and a protective layer. The light reflective layer usually comprises particles of a light reflecting substance, e.g., baryta or titanium dioxide in a polymeric binder. The fluorescent layer usually comprises particles of a fluorescent substance for example calcium tungstate, in a polymeric binder. In order to protect the surface of the fluorescent layer a protective layer which is usually a transparent polymeric substance is coated on to the fluorescent layer. If the dimensionally stable support used is transparent, forinstance'if polyester is used, it is not necessary for a light reflecting layer to be present between the base support and the fluorescent layer. In this case the light reflective layer may be present on the side of the base support distal to the fluorescent layer or separate light reflecting means may be used in conjunction with the intensifying screen.

When the X-ray intensifying is a so-called salt intensifying screen that is to say the fluorescent layer comprises particles of a metal salt, e.g., calcium tunstate in a binder, it is preferred that the ratio of salt to binder should be as high as possible. Thus the quantity of polymeric binder must be sufficient to hold the powder together and impart adequate layer strength, but on the other hand as little as possible should be present in order to reduce fluorescent light absorption by the binder. However when a coating mixture of a fluores! cent salt, a polymeric binder and coating solvents, which has just sufficient binder to impart adequate strength to the mixture, is cast as a layer the coating solvents will evaporate and the layer will dry out to form a layer which has many air spaces therein. When the protective layer which comprises a transparent polymeric material dissolved in an organic solvent or solvents is coated on to a layer of this type having many air spaces therein, it has been found that the organic solvent penetrates into the underlying fluorescent layer. When during manufacture the material goes into the drying area a protective skin is rapidly formed, thus trapping the solvent which has penetrated to the fluorescent layer. As the coated material progresses through the drying area the trapped solvent tends to break through the skin surface of the protective layer causing permanent marks of eruption in this layer. Further, sometimes air entrapped in the fluorescent layer is also released, thus causing further eruption marks in the protective layer.

It is the object of the present invention to provide a method of preparing X-ray intensifying screens wherein as little polymeric binder as possible is used in the salt fluorescent layer and wherein trouble caused by the solvent used in the protective layer is minimised.

According to the present invention there is provided a method of preparing an X-ray intensifying screen of the salt type which comprises coating on to a dimensionally stable base a dispersion of a fluorescent salt in a low-boiling point organic coating solvent or solvents having dissolved therein a polymeric binder together with a high-boiling solvent selected from those which have an evaporation rate on the ether scale above 35 but below 200, allowing or causing the coating solvent Evaporation rate of the solvent Time for evaporation of the solvent Time for evaporation of diethyl ether the evaporation times being determined under the same experimental conditions.

Preferably the protective layer, that is to say the transparent film-forming polymeric material dissolved in an organic solvent is coated on to the still moist fluorescent layer by spraying means, e.g., by using a spray gun.

The coating solvent or solvents employed evaporate very quickly from the coated fluorescent layer and usually it is not required that the coated base be heated in order to remove the coating solvents. As the coating solvents evaporate the binder solidifies and the fluorescent salt plus binder forms a honeycombe-like structure. In the process of the present invention this honeycombe-like structure will be full of the high-boiling point organic liquid and not full of air holes. Thus when the protective layer is coated thereon the solvent in this coating cannot penetrate the fluorescent layer as it encounters an apparently fully bound layer. Thus it is an essential feature of the present invention that the protective coating is applied to a coated fluorescent layer while this layer is still moist and before the high-boiling solvent has evaporated from this layer.

Any of the usual fluorescent salts may be used in the process of the present invention, for example calcium tungstate, zinc sulphide and barium sulphate/lead sulphate. Suitable polymeric binders are for example cellulose nitrate, cellulose triacetate, cellulose acetate/butyrate and polyvinyl compounds for example polyvinyl butyral. It is preferred that in the coating mixture used for the fluorescent layer the amount of polymeric binder present is just sufficient when dried to ensure adequate strength of the layer. The weight of fluorescent salt compared with the binder is preferably within the region of of salt to l of binder, to 25 of salt to l of binder (by weight). A particularly suitable ratio is 33 of salt to l of binder. If more than 40 of salt to l of binder is employed the resultant screen has a rather weak structure. If less than 25 of salt to l of binder is employed the definition of the screen is reduced. When about 33 of salt to l of binder is employed it has been found that an approximately equal weight of highboiling point solvent is required in the coating mixture to ensure that when the layer is dried that all the spaces in the honeycombe structure of the fluorescent salt plus binder are filled with this liquid.

Examples of suitable low boiling point coating solvents (aid solvents) are acetone and a mixture of acetone and ethyl acetate.

The protective layer may be any of the usual polymeric compounds employed for this purpose for example cellulose acetate, cellulose nitrate or polymethyl methacrylate.

There may be present in the fluorescent salt coating layer a-plasticizer for example dibutyl phthalate.

It is to be understood that a light reflective layer which comprises light reflective particles in a polymeric binder may be cast first on to the dimensionally stable support before the fluorescent layer is cast on to this support. If the base support is' transparent the fluorescent layer may be coated on to the other side of the support to the light reflecting layer. The preferred base is polyester and it is usually required that the polyester base is comparatively thick compared with a base used for photographic purposes that is to say the base used should be in the region of 0.025 cm.

The preferred solvent to be coated with the fluorescent layer is ethyl lactate (evaporation rate 80). Other suitable solvents are, for example, diacetone alcohol (e.r. 150), ethylene glycol monoethyl ether (e.r. 45) and ethylene glycol monoethyl ether acetate (e.r. 52).

It is important that the drying process is a controlled process because not only does the solvent in the protective layer evaporate but also the high-boiling point solvent used in the fluorescent layer slowly evaporates through the protective layer. It has been found that slow evaporation of the high-boiling point solvent through the protective layer does not cause any eruption in the protective skin which forms rapidly on the protective layer. Further, no air is entrapped in the fluorescent layer and thus cannot be liberated by the drying process. Thus it is an important feature of the present invention that the evaporation rate on the ether scale of the selected solvents is between and 200. Solvents having an evaporation rate below this figure evaporate too quickly from the fluorescent layer causing eruptions in the skin formed on the protective layer. Solvents which have an evaporation rate higher than 200, e.g., dibutyl phthalate will not evaporate appreciably from the fluorescent layer. In fact a certain amount of dibutyl phthalate is often used as a plasticiser in the polymeric binder used to bind the fluorescent salts. However most of the dibutyl phthalate remains in this layer after the drying treatment.

It has been found that spray coating the protective layer is by far the best method of coating the protective layer onto the still moist and thus soft fluorescent layer. The spray coating method has the advantage that it is quick and can be carried out in the same pass of the coating apparatus as the coating of the fluorescent layer on to the material.

The following example will serve to illustrate the preparation of an X-ray intensifying screen made by the method of the present invention.

EXAMPLE The support base for the screen was a web of 0.025 cm. polyester base coated with a subbing layer. In a first pass through the coating apparatus a reflective coating which comprised titanium dioxide dispersed in polyvinyl acetate (as binder) dissolved in an acetone- /absolute alcohol mixture was applied to the web and the coating was dried. The dried web was then passed through the coating apparatus for a second time and a fluorescent layer having the formulation set forth below was coated thereon.

Fluorescent layer:

Calcium tungstate (fluorescent powder) l200 g Cellulose nitrate (binder for fluorescent powder 22.5% solution in acctonc) 1440 g Acetone (coating aid solvent) 950 ml Ethylacetate (coating aid solvent) 780 ml Ethyl lactate (high boiling point solvent) 600 ml Dibutyl Phthalate (plasticiser) 480 ml Then while this fluorescent layer was still moist there was sprayed on to the fluorescent layer a protective layer having the following formulation:

Protective layer:

Cellulose acetate 800 g Acetone (solvents for 8750 mls Absolute alcohol cellulose 970 mls Toluene acetate) 4860 mls Methyl Cello-Solve Acetate 5220 mls Methyl Phthallyl Ethyl Glycollate (plasticizer) l20 mls The web was then stored at a slightly elevated temperature. This ensured that all the ethyl lactate had diffused out of the coated web before the web was cut into discrete screens.

I claim:

1. A method of preparing an X-ray intensifying screen of the salt type which comprises applying a dispersion of a fluorescent salt in a low-boiling point organic coating solvent or solvents solution of a polymeric binder together with a high-boiling point solvent selected from those which have an evaporation rate on the ether scale above 35 but below 200 to a dimensionally stable base, evaporating the low-boiling point organic coating solvent or solvents, applying a transparent film-forming polymeric material dissolved in a solvent to the fluorescent layer containing the high-boiling point solvent and while this layer is still moist and evaporating the high-boiling solvent still present in the fluorescent salt layer through the overlaying protective layer, and drying the X-ray intensifying screen so produced.

2. A method according to claim 1 which comprises coating the solution of the transparent film-forming polymeric material on the fluorescent layer by use of a spray gun.

3. A method according to claim 1 which comprises coating initially a reflecting layer on the dimensionally stable base and after this layer has been dried coating thereon the dispersion of a fluorescent salt in a polymeric binder together with a high-boiling solvent.

4. A method according to claim 1 which comprises selecting the high-boiling solvent having an evaporation rate on the ether scale above 35 but below 200 ethyl acetate.

'from the group consisting of ethyl lactate, diacetone An X43}, intensifying Screen Ofthe Salttype which alcohol, ethylene glycol monoethyl ether and ethylene glycol monoethyl ether acetate.

5. A method according to claim 4 wherein the low- 5 boiling solvent is acetone or a mixture of acetone and has been prepared by the method as claimed in claim 

1. A METHOD OF PREPARING AN X-RAY INTENSIFYING SCREEN OF THE SALT TYPE WHICH COMPRISES APPLYING A DISPERSION OF A FLUORESCENT SALT IN A LOW-BOILING POINT ORGANIC COATING SOLVENT OR SOLVENTS SOLUTION OF A POLYMERIC BINDER TOGETHER WITH A HIGHBOILING POINT SOLVENT SELECTED FROM THOSE WHICH HAVE AN EVAPORATION RATE ON THE ETHER SCALE ABOVE 35 BUT BELOW 200 TO A DIMENSIONALLY STABLE BASE, EVAPORATING THE LOW-BOILING POINT ORGANIC COATING SOLVENT OR SOLVENTS, APPLYING A TRANSPARENT FILM-FORMING POLYMERIC MATERIAL DISSOLVED IN A SOLVENT TO THE FLUORESCENT LAYER CONTAINING THE HIGH-BOILING POINT SOLVENT AND WHILE THIS LAYER IS STILL IN THE FLUORESCENT SALT LAYER THROUGH T SOLVENT STILL PRESENT IN THE FLUORESCENT SALT LAYER THROUGH THE OVERLAYING PROTECTIVE LAYER, AND DRYING THE X-RAY INTENSIFYING SCREEN SO PRODUCED.
 2. A method according to claim 1 which comprises coating the solution of the transparent film-forming polymeric material on the fluorescent layer by use of a spray gun.
 3. A method according to claim 1 which comprises coating initially a reflecting layer on the dimensionally stable base and after this layer has been dried coating thereon the dispersion of a fluorescent salt in a polymeric binder together with a high-boiling solvent.
 4. A method according to claim 1 which comprises selecting the high-boiling solvent having an evaporation rate on the ether scale above 35 but below 200 from the group consisting of ethyl lactate, diacetone alcohol, ethylene glycol monoethyl ether and ethylene glycol monoethyl ether acetate.
 5. A method according to claim 4 wherein the low-boiling solvent is acetone or a mixture of acetone and ethyl acetate.
 6. An X-ray intensifying screen of the salt type which has been prepared by the method as claimed in claim
 1. 